The invention is particularly applicable for use in a fireplace insert of the type now commonly sold for use in an existing fireplace, and it will be described with particular reference thereto; however, the invention has brought applications and may be used in free-standing woodburning stoves of which there are many examples. Such stoves are shown in Gullickson U.S. Pat. Nos. 4,316,444 and Frank 4,232,650.
In recent years, there has been a tremendous growth in the popularity of woodburning stoves, either of free-standing type or of the type located within a fireplace as a replacement for the inefficient fireplace normally provided in domestic dwellings. In view of this significant usage of such woodburning appliances, manufacturers have been attempting to increase the efficiency so that the energy produced by the burning wood is available for heating a room in which the appliance is located. In the past, domestic fireplaces were highly inefficient and most heat energy passed upward through the chimney. To increase the efficiency, the firebox or combustion chamber was enclosed and room air was circulated around bonnets or other heating surfaces within the stove to use the heat from the burning wood for increasing the temperature of the air circulating through the woodburning device. In this manner, the energy from the woodburning within the firebox was transmitted to the circulated room air for the purpose of increasing the efficiency of standard fireplaces. Even with these air circulating units, the burning efficiency of the wood itself was relatively low and a substantial amount of burnable or combustible particulate material passed upwardly, from the combustion chamber through the exhaust flue opening by normal draft. Consequently, the overall efficiency involving the total energy available in the wood was contingent upon this efficiency of burning within the firebox and the efficiency of transferring the heat from the firebox into the room. Heating efficiency can be increased by faster and hotter burning of the wood; however, this produces excessive heat which is not needed in the room during a relatively short time necessary to consume the fast burning wood. For that reason, woodburning stoves and fireplaces are usually throttled down at the primary air inlet to burn the wood fairly slowly. When this occurs, the burning efficiency drops and substantial particulate material passes up through the flue and into the atmosphere. Such reduced efficiency is an economic disadvantage in that the stove or insert consumes more wood. As a secondary consideration, slow burning of the wood substantially pollutes the atmosphere. Federal and State agencies are now promulgating regulations which require low pollution levels for fireplace inserts and stoves of the woodburning type. In 1988, the State of Oregon will require that such devices have an exhaust containing no more than 9.0 gr/hr particulate material as a weighted average. Other states are considering similar requirements for woodburning stoves and fireplace inserts.
Rigid State and impending Federal regulations will make it imperative that a woodburning stove or fireplace insert have a weighted average particulate value of less than about 9.0 gr/hr, such as is now the 1988 standard in the State of Oregon. Since manufacturers of fireplace inserts cannot guarantee where there units will be used or sold and do not want to exclude sales in any region of the country, there is a tremendous effort in the woodburning fireplace insert and stove industry to develop units which can pass or be certified in all states, including a state having a particulate maximum of about 9.0 gr/hr. The most convenient approach by manufacturers is to employ a catalytic converter, as shown in Allier U.S. Pat. No. 4,330,503. Many manufacturers are retrofitting or redesigning their standard stove or fireplace inserts to employ one of these converters. This approach is convenient, but expensive. Indeed, converters do not accomplish the real intent of the State and Federal regulations. A fireplace, stove or insert having a separate catalytic converter must have a flue gas bypass, so that the fire in the firebox reaches a certain temperature before the flue gases are passed through the catalytic converter. There is no assurance that a user of the insert will operate the bypass or will even understand its operation. In addition, if the bypass is not operated, the catalytic converter can become damaged or clogged and in such condition, the converter will not function to reduce particulate material. If the user burns toxic material containing certain minerals, the catalytic converter is immediately destroyed or its effect diminished. As can be seen, many things can occur which will make the catalytic converter inoperative for the purposes of controlling air pollution. The catalytic converter is quite expensive and available from a very limited number of sources; therefore, when it becomes inoperative, a user of the fireplace insert can remove the catalytic converter or continue to use the inoperative converter. All of these faults with converters make the use of a catalytic converter counterproductive for the purposes of reducing air pollution. Claims by manufacturers regarding transfer and burning efficiencies are diminished as converters become ineffective. Thus, although converters are convenient and are available to manufacturers wanting to avoid impending decertification, these catalytic converters are not necessarily the total answer to the problem of high efficiency and/or reduced air pollution.
Assuming that a manufacturer has decided not to incur the expense of a catalytic converter, either as original equipment or retrofit onto his present design, its inserts or woodburning stoves must be redesigned to meet the new standards which standards are determined by testing the particulate material issuing from the unit through at least four separate burning ranges controlled by the amount of primary air available to the firebox. These ranges are less than 10,000 BTU/hr; 10,000-15,000 BTU/hr; 15,000-20,000 BTU/hr; and over 20,000 BTU/hr. These tests are costly and are conducted for certification by approved testing organizations.
Since most domestic fires are at low burning rates, the most critical and heaviest weighted range is 10,000 BTU/hr or less. Manufacturers not opting for catalytic converters or catalytic converter rectofits have not been able to meet the 9.0 gr/hr 1988 Standard for Oregon; therefore, to be certified, the stoves or inserts have been modified to limit the inlet for primary air to a minimum amount causing rates at least substantially over 10,000 BTU/hr. In other words, since the problem most manufacturers face is heavy particulate at extremely low burning rates, their units must have a preselected minimum air getting to pass the impending air pollution standards. This concept of assuring only rapid burning of the wood to limit particulate exhaust is illusary. Such fireplace inserts or stoves sold to customers will burn too hot for normal domestic use. Consequently, these hot burning stoves and inserts create dissatisfied consumers and motivate users to block off the incoming primary air by separate devices like tape or by modifying the original equipment to allow greater reduction in the primary air. Thus, fireplace inserts presumably meeting certain air pollution standards, due to manufacturered minimum air capabilities, often are modified to burn at the lower air rates. This produces inefficiency and air pollution.
In view of this totally chaotic situation in reducing air pollution by fireplace inserts, there is a substantial need and desire among the manufacturers of stoves and fireplace inserts to develop a clean burning, high efficient unit which can be manufactured at a competitive price and does not involve a catalytic converter.